No one should object to President Bush’s tapping of domestic phone calls and e-mail messages when the other end of the conversation appears to be a known al-Qaida agent, if the president had acted within the law. That sort of eavesdropping might have alerted him to the 2001 terrorist attacks, and it might tip him off to some new attack.

Nor should anyone complain about the secrecy of such a sensitive intelligence program. Secrets are often essential, especially in wartime. Even Mr. Bush’s earlier misstatement of the facts may be excused, since he was trying to keep the secret. He said in April 2004, in a discussion of the Patriot Act: “A wiretap requires a court order. Nothing has changed, by the way. When we’re talking about chasing down terrorists, we’re talking about getting a court order before we do so.”

But, now that the president is openly defending warrantless wiretapping in the face of a law that requires warrants, the Senate Judiciary Committee is rightly planning oversight hearings to learn details of the eavesdropping. The committee chairman, Sen. Arlen Specter, a Pennsylvania Republican, has said that the committee wants to know “the statutory or other legal basis for the electronic surveillance, whether there was any judicial review involved, what was the scope of the domestic intercepts, what standards were used to identify al- Qaida or other terrorist callers, and what was done with this information.”

New facts have come out since the original story in The New York Times, suggesting a further subject for the committee’s inquiry. Beyond the reported 500 domestic intercepts at any one time, officials later told of a vast data-mining operation covering far more conversations than the White House has acknowledged.

With such a powerful tool in its hands, the Bush administration has aroused suspicions. For example, administration officials have expressed interest in learning who informed Times reporters about the eavesdropping program. The Times and The Washington Post and other news organizations have demonstrated that they have good confidential sources inside the government. The question naturally has arisen whether eavesdropping is being employed to stop leaks.

Spying on American citizens would be nothing new. The Nixon administration set up a secret task force known as “The Plumbers” in an effort to plug leaks and to spy on people on its “enemies list.” The exposure of that earlier eavesdropping operation figured in the Watergate scandal.

The Judiciary Committee should find out which Americans’ wires have been tapped. As it conducts its oversight hearings, it must, of course, respect remaining secret details of the operation. The public can hardly expect to see lists of Americans subjected to the eavesdropping. But the committee should insist on learning whether the warrantless surveillance program has been used not only as a tool to protect against global terrorism, but also to snoop on some new enemies list.